sort.cc: introduce gcc_sort_r

sort.cc: introduce gcc_sort_r

	* sort.cc (sort_r_ctx): New struct.
	(reorder23): Make templated on context type.
	(reorder45): Ditto.
	(cmp1): Ditto.  Adjust signature.
	(netsort): Ditto.
	(mergesort): Ditto.
	[CHECKING_P] (cmp2to3): New static function.  Use it...
	(gcc_qsort) [CHECKING_P]: ...here.
	(gcc_sort_r): New function.
	* system.h (sort_r_cmp_fn): New function typedef.
	(qsort_chk): Adjust signature.
	(gcc_sort_r): Declare.
	* vec.c (qsort_chk_error): Adjust.
	(qsort_chk): Adjust.

From-SVN: r273977